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  • Estimation of Towing Forceson Oil Spill Containment Booms

    prepared by

    SL Ross Environmental ResearchOttawa, Ontario, Canada

    prepared for

    Minerals Management ServiceHerndon, Va, U.S.A.

    July 1999

  • -i-

    Acknowledgments

    This report was prepared by Stephen Potter and James McCourt of SL Ross Environmental Research

    Limited.

    The authors gratefully acknowledge: the United States Coast Guard, the U.S. Navy Supervisor of

    Salvage, and the Canadian Coast Guard for the use of their containment booms used in this study;

    the staff at Ohmsett for their advice and assistance in planning and carrying out the tests; and the

    Minerals Management Service and the United States Coast Guard for funding this study.

  • -ii-

    Table of Contents

    Acknowledgments

    1. Introduction...............................................................................................................................1

    2. Objectives .................................................................................................................................2

    3. Previous Work ..........................................................................................................................3

    3.1 Existing Formulae for Estimating Tow Forces.........................................................................3

    3.2 MSRC / USCG Testing of Containment Booms ......................................................................6

    4. Test Methodology.....................................................................................................................7

    4.1 Test Facility and Equipment .....................................................................................................7

    4.2 Containment Booms Tested......................................................................................................7

    4.2 Test Variables .........................................................................................................................10

    5. Results.....................................................................................................................................12

    5.1 Comparison of Results with Formula Predictions ..................................................................12

    5.2 Comparison of Results with Field Testing .............................................................................12

    5.3 Correlation of Results .............................................................................................................17

    5.4 Grouping of Results by Boom Size and Type.........................................................................19

    6. Conclusions.............................................................................................................................21

    7. Recommendations...................................................................................................................22

    8. References...............................................................................................................................23

    Appendix A: Specifications and Calibration Curves of Load Cells

    Appendix B: Summary of Recorded Tow Force Values per Boom

    Appendix C: Tow Force vs. Tow Speed Curves per Boom

  • -iii-

    List of Figures

    Figure 1: Tension parameter ( ) vs. gap ratio ................................................................................5

    Figure 2: Layout of Ohmsett tank with boom in position for towing ..............................................8

    Figure 3: Tow force data, calm conditions, comparing

    measured values with formula predictions ...............................................................13

    Figure 4: Tow force data, regular waves, comparing

    measured values with formula predictions ...............................................................14

    Figure 5: Comparison of MSRC field test data with test tank data ...............................................15

    List of Tables

    Table 1: Tension parameter ( ) for selected gap ratios ..................................................................4

    Table 2: Summary of containment booms tested.............................................................................9

    Table 3: Summary of wave conditions used ..................................................................................10

    Table 4: Comparison of data with MSRC tests .............................................................................16

    Table 5: Value of constant K, for various booms ..........................................................................18

    Table 6: Value of constant K', for various booms..........................................................................19

    Table 7: Recommended size of boom per water body...................................................................20

    Table 8: Values of constant K' for booms grouped according

    to water body classification ......................................................................................20

  • -1-

    1. Introduction

    Effective use of skimmers or in situ burning for an oil spill generally requires that the spill first be

    contained using booms. Typically, a containment boom would be towed in a "U" configuration or

    held stationary against a current in order to contain and thicken oil for recovery or burning. In either

    case, it is important to know the likely forces imposed on a boom so that appropriately sized tow

    vessels and towing gear are specified for the operation, and more important, so that boom with

    sufficient tensile strength is selected. Guidance for selecting appropriate tensile strength is provided

    in U.S. Coast Guard 33 CFR Part 155, Vessel Response Plans Final Rule (USCG 1996), and in

    ASTM F1523: Selection of booms in accordance with water body classifications (American Society

    of Testing and Materials 1996).

    Presently, boom towing forces are estimated using several well-known formulae such as those

    published in the World catalog of oil spill response products (Schulze 1995), Exxon oil spill field

    manuals (Exxon 1982), and International Tanker Owner's Pollution Federation (ITOPF) field

    manuals (ITOPF 1986). These formulae estimate the theoretical loads on a boom based on its

    dimensions, water current or tow speed, wave height, and wind, and include constants to account for

    boom profile and gap ratio. Recent field testing carried out for the Marine Spills Response

    Corporation (MSRC) and the U.S. Coast Guard (USCG) (Nordvik et al. 1995a) has shown that these

    formulae may severely underestimate drag forces. As a result, commonly accepted values for the

    minimum required tensile forces in a boom may be well below the actual required values.

    A series of tests was carried out at the Ohmsett test facility to measure the towing forces on a number

    of booms using a range of gap ratios, wave conditions, and tow speeds. The data from these

    experiments was used to develop a simple relationship to predict the tow force and required tensile

    strength for the various boom and tow parameters. A comparison was also made between the tow

    forces as measured in the Ohmsett test tank against those measured in the MSRC / USCG field

    testing.

  • -2-

    2. Objectives

    The objective of the study was to determine the loads developed on a containment boom when towed

    in a typical operational configuration. The work was conducted in four phases:

    a test protocol was prepared and circulated for comment among the project participants;

    equipment for testing was identified and assembled at Ohmsett;

    the tow tests were carried out at Ohmsett in July 1998;

    the results were analyzed and the following report prepared to document the study; and,

    the results were presented to the ASTM F20 Committee and at the Arctic and Marine

    Oilspill Program (AMOP) Technical Seminar 1999.

  • -3-

    3. Previous Work

    The first phase of the work was a brief review of recent boom testing that included the determination

    of towing forces. The goal was to establish the theoretical validity of existing formulae given

    modification to the constants used for boom shape and gap ratio.

    3.1 Existing Formulae for Estimating Tow Forces

    The formulae currently used for predicting tow loads on containment boom include the following.

    The Schulze formula is known as such as it is published in the World Catalog of oil spill response

    products (Schulze 1995). It was originally published in an Exxon spill manual (Exxon 1982), and

    is based on a theoretical consideration of the wind and current forces acting on a boom. The formula

    is as follows:

    Ta = 0.5 L Cd a f Va2

    Tw = 0.5 L Cd w d (Vw + 0.5 Hs)2

    D = 2 (Ta + Tw)

    where: D = total drag force, lbfTa = tension due to wind, lbfTw = tension due to waves and current, lbfVa = wind speed, ft/sVw = current/tow speed, ft/sa = density of air (0.00238 slugs/ft

    3)w = density of water (1.98 slugs/ft

    3)L = length of boom, ft = tension parameter, dimensionlessCd = drag coefficient [assumed to be 1.5], dimensionlessf = boom freeboard, ftd = boom draft, ftHs = significant wave height, ft

  • -4-

    It is interesting to compare the effects of wind and water currents on the total load imposed on a

    boom. For example, using this formula, and assuming that the freeboard dimension is half the draft

    (which is typical of containment boom), and assuming a 20 knot

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